Commercial devices which effectively handle suspensions, such as paper pulp, at medium consistency, that is at about 6-15% solids consistency, are known. It is also known that air or, more generally gas, if present in the fiber suspensions causes problems in almost all process stages in the pulp and paper industry. When pulp is pumped, mixed, screened, washed or otherwise handled without excess gas significant savings in equipment, power consumption and the like can be achieved. For instance, one device which has been particularly successful in allowing handling of gas-containing medium consistency fiber suspensions is a fluidizing centrifugal pump which simultaneously pumps and degasses the suspension. Typically, such pumps utilize a separate vacuum pump, piping from the centrifugal pump to the vacuum pump, a separate motor and motor mount for the vacuum pump, etc., in order to exhaust the gas which has been separated from the suspension so that the suspension may be effectively pumped by the pump impeller.
U.S. Pat. No. 3,230,890 discloses a centrifugal pump for removing gas from low consistency suspensions or from water having either a built-in vacuum pump or an external vacuum pump.
A fluidizing centrifugal pump having a built-in vacuum pump is disclosed in U.S. Pat. No. 4,776,758. The vacuum pump is a so-called liquid ring pump which has been arranged on the same shaft as the impeller behind an intermediate plate separating the centrifugal pump chamber from the vacuum pump chamber. A narrow ring-shaped duct serving as the gas inlet to the vacuum pump is arranged in the intermediate plate. The gas outlet, however, is provided in the back of the vacuum pump housing which contains the vaned rotor with a cylindrical central portion mounted for rotation between the front and back wall within the vacuum pump chamber.
Various problems have, however, been encountered with the pump in operation today. For example, the air removal capacity has been significantly lower than required, i.e. the vacuum created has not reached a sufficiently high level. Also, the discharge pressure of the vacuum pump has been found to be too low. In some cases, the material discharged from the vacuum pump, a mixture containing mainly gas but also some fibers, has been introduced into the top portion of a mass tower to recover the fibers. If, however, the discharge pressure of the vacuum pump is too low the pumped material cannot be conveyed to the top of the mass tower, and an additional pump must be installed for that purpose. Also, the open annular volume in the intermediate plate of prior art pump has a tendency to become clogged by the fibers.
In the prior art pump the axial gap between the vanes of the vacuum pump and the axially adjacent walls of the vacuum pump housing are not adjustable but are positioned at a distance or clearance of about 0.4 mm. The reasons for such relatively large clearance is the fact that there are a number of factors which render it is impossible to further decrease the clearance as the various components of the pump are installed on the shaft or around the shaft starting from the drive end of the shaft. Thus, the dimensions of the components effect the clearance. The result of too wide a clearance is, of course, an insufficient vacuum. Another reason for the wide clearance may also be the fact that the shaft of the pump tends to flex somewhat during operation creating the risk of mechanical contact between the vacuum pump vanes and the housing walls. Thus, the large clearance has been provided intentionally to ensure long lasting operation of the pump.
The pump in accordance with the present invention is designed to eliminate most or all of the above problems. Accordingly the pump of the present invention is provided with an intermediate plate separating the centrifugal pump housing from the vacuum pump which is, preferably, a liquid-ring pump. Within this intermediate plate a gas discharge duct is provided which permits exhaustion of air from the vacuum pump. The air which is collected in front of the impeller will initially travel through one or more openings provided in the impeller backplate towards the back of the impeller and from there through an opening and non-annular volume which are also located within the intermediate plate to the suction side of the vacuum pump. As the rotor continues to rotate, the gas present between the vanes of the rotor is compressed by the liquid ring and is expelled through the discharge port located also within the intermediate plate.
Preferably, the rotor central portion has a linearly varying diameter so that it is preferably conically tapered from the back wall of the pump toward the gas inlet and gas discharge port for preventing the build-up of air pockets around the rotor central portion. The vacuum pump rotor may also be provided with a radial extension forming a rotor shroud at the side opposite the air inlet and air discharge openings and which has preferably the same height as the rotor vanes. The shroud may be provided with a plurality of openings for the entry of make-up air for the control of the vacuum pump which is further explained below. The make-up air may, however, be introduced into the vacuum pump from the opposite side of the vacuum pump, i.e. from the side of the gas inlet and gas discharge ports so that, in this case, the rotor shroud may be entirely solid without openings therethrough.
The pump of the present invention may also be provided with means for introducing a liquid into the pump, and especially into the non-annular volume in the intermediate plate and air flow ducts of the pump for flushing these critical locations with a liquid such as flushing water and freeing the pump from fibers which otherwise tend to block the flow path of the pump. The flushing ducts may also be used to supply working liquid to the liquid ring of the vacuum pump.
The pump of the present invention also provides means for adjusting the relative axial position of the vacuum pump rotor relative to the front and rear wall of the vacuum pump chamber thereby providing significantly smaller operational of clearances therebetween. This may be achieved by either adjusting the axial position of the rotor with respect to the shaft, for example, by the addition of preferably annular shims between respective shoulders of the vacuum pump rotor and shaft. The relative axial position of the vacuum pump rotor with respect to the vacuum pump chamber may also be optimized by adjusting the axial position of the shaft with respect to the vacuum pump chamber and the centrifugal pump body, in which case the vacuum pump rotor is fixedly attached to the shaft. Finally, the relative axial position of the vacuum pump rotor and the vacuum pump chamber is optimized by adjusting the vacuum pump chamber with respect to the rotor and the centrifugal pump body.
In addition, a port for the admission of make-up air for the control of the vacuum pump may be provided at the rear wall of the vacuum pump. By rear wall of the vacuum pump is meant that wall which is located opposite the air inlet port remote from the intermediate plate. As pointed out above, if the rear wall of the vacuum pump is formed at least in part by a circumferential wall or shroud which radially extends from the rotor central portion, a plurality of openings are provided within the radial wall preferably in register with make-up air inlet ducts within the vacuum pump housing for permitting the air to enter the vacuum pump.
Axial clearances between the vacuum pump rotor and the vacuum pump chamber walls may also be adjusted by providing a rotor with rotor blades which are slightly tapered in radial direction or by providing a vacuum pump chamber wherein the side walls are slightly tapered in radial direction relative to the shaft to account for the slight bending or flexing of the shaft during operation of the vacuum pump.
The pump of the present invention may also be provided with means for introducing a sealing liquid to the clearances between the vacuum pump rotor and adjacent side walls for sealing the same and thus increasing the pumping action of the device. The sealing liquid may be introduced separately to one or both sides of the vacuum pump chamber so that it can flow into and seal the space or clearance between the pump rotor and adjacent side walls of the vacuum pump. The sealing liquid may also be fed to the spaces through a single conduit leading through the central portion of the vacuum pump rotor. A control valve for regulating the vacuum of the vacuum pump may also be directly attached at the end of the make-up air channel.